Hydrocarbon isomerization processes are widely used to convert normal hydrocarbons to more valuable non-normal hydrocarbons. The more valuable non-normal hydrocarbons may be used as gasoline blending components to boost the octane number of the gasoline or as intermediates for such oxygenate products as methyl tertiary butyl ether, ethyl tertiary butyl ether, and tertiary amyl ethyl ether. One class of vapor phase hydrocarbon isomerization processes uses adsorption technology to remove non-isomerized normal hydrocarbons from the isomerization reactor effluent. The adsorbed normal hydrocarbons are desorbed using hydrogen and recycled to the isomerization reactor. The overall production of the process is enhanced by keeping reactants in circulation within the process until the desired products are formed. Typically this class of isomerization processes is referred to as total isomerization processes, or TIP. Detailed descriptions of variations of this isomerization technique may be found in Crusher, N. A. In Handbook of Petroleum Refining Processes 2.sup.nd ed.; Meyers, R. A. Ed.; McGraw-Hill: New York, 1997; pp 9.29-9.39, U.S. Pat. No. 4,210,771, U.S. Pat. No. 4,709,117, and U.S. Pat. No. 4,929,799 which are all incorporated by reference.
As with most processes, significant operational costs to be considered in total isomerization processes include the cost of utilities. Innovations to reduce the utilities' costs are continuously sought and those that are successful can greatly improve the economics of the process. Heat integration is one technique used to reduce utilities' costs. U.S. Pat. No. 4,210,771 discloses using specific heat integration in an isomerization process such as described above. The reference discloses a variable fresh feed that is combined with the desorption effluent from an adsorbent bed used to adsorb normal hydrocarbons. The combined streams are then flowed to an isomerization reactor. Before the adsorbed normal hydrocarbons elute from the adsorbent bed, an adsorption effluent containing non-normal hydrocarbon product is withdrawn from the adsorbent bed. The adsorption effluent is heat exchanged with the fresh feed to partially heat the fresh feed before it is combined with the desorption effluent and introduced into the reactor.
Utility consumption and therefore cost has also been reduced by new catalysts developed for use in the isomerization reactor; see U.S. Pat. No. 5,036,035 and U.S. Pat. No. 5,120,898 both of which are incorporated by reference. Traditional temperatures in the isomerization reactor have been in the range of 245.degree. C. to 370.degree. C., but newer catalysts are efficient at significantly lower temperatures such as from about 70.degree. C. to about 250.degree. C.; see U.S. Pat. No. 5,120,898, incorporated by reference. However, current adsorbents used in the adsorption zone require that the adsorption zone continue to be operated at the higher temperatures in order to prevent capillary condensation in the adsorbent pores.
The newer catalysts and the resulting temperature variation between the adsorption zone and the isomerization reactor have provided the opportunity for enhanced heat integration. The temperature of the stream carrying normal hydrocarbons from the adsorption zone to the isomerization reactor, the desorption effluent, must be reduced from the operating temperature of the adsorption zone to the operating temperature of the isomerization reactor while at the same time, the reactor effluent is cooled and separated to form an adsorber feed stream and a hydrogen purge gas each of which must be increased to that of the adsorption zone. The present invention reduces the utility costs of the overall process through heat exchanging the adsorber feed and/or the hydrogen purge gas with the desorption effluent. However, because the desorption effluent is not a constant mass flow stream, traditional heat exchanging techniques are insufficient. The present invention requires traditional heat exchanging techniques to be used in conjunction with a variable steam or hot oil flow or in conjunction with a surge drum.